Polymeric foams are materials with high importance in the field of composite materials, and are used for many applications, e.g. for insulation, structural parts such as car dash boards, as well as for core materials in manufacturing of composite sandwich panels exhibiting high strength, improved energy dissipation, insulation, and light weight. Polymeric foams convey high insulation and weight reduction properties; however, some have low strength.
Sandwich structure composites can be described by thin and stiff facings that are attached to light weight thick core. The core material is normally limited strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density. Combination of the properties of the facings and the core results in structures which are extremely light and strong.
Materials used for core in sandwich structures can be divided into rigid foams or honeycomb structures. Expanded PVC and PET foams are examples for commonly used rigid foams. These foams are produced by chemical blowing (foaming) of the polymers and form sponge like rigid isotropic structures. Honeycomb structures such as aluminum, Kevlar, polypropylene, or cardboard form anisotropic structures with high compressive and shear strengths in the Z axis of the structure. Honeycomb structures, which are anisotropic by nature, are very useful as core for composites. Such cores resist the shear loads, increase the stiffness of the structure by holding the facing skins apart, and provide continuous support to flanges or facing skins, thereby producing uniformly stiffened structures.
Recently, it was shown that nanocrystalline cellulose (NCC) as well as nano-fibers can be processed into foams by various methods. NCC is a fibrous material produced from cellulose, typically being high-purity single crystals having an elongated shape. These constitute a generic class of materials having mechanical strengths equivalent to the binding forces of adjacent atoms. The resultant ordered structure is typically characterized by high mechanical strengths; the tensile strength properties of NCC are far above those of currently available high volume content reinforcements [1-3].
Irrespective of the processes employed for producing these foams, such as supercritical fluid extraction, micro-fluidics, etc., these foams display low resistance for compression and therefore their utilization as core materials is limited [3].
One available method for cellulose based foam formation is called “foam forming”, according to which method cellulose pulp fibers are mixed with a detergent and the foam is thereafter produced following standard papermaking methods on a paper machine. The resulting product is a light weight flexible and soft foamed paper sheet with microporous structure and low degree of orientation.
Another available method for production of cellulose foams involves casting a NCC or a nano-fiber suspension into molds followed by freeze-drying [4-10]. Such foams self-assemble due to ice formation which pushes the NCC particles one towards each other. Consequently, controlling the ice growth results in controlled patterning of the foams This process, termed “ice-templating”, was developed for controlling the assembly of a variety of materials and is typically used for assembly of colloidal suspensions into solids. To date, use of ice-templating was tested in laboratory-scale, and was found to yield foams of significantly inferior properties as compared to rigid synthetic foams.
Common freezing processes, as mentioned above, may result in the formation of light weight but very soft, easily disintegrated and low compressive strength structures, mainly due to the difficulty in controlling the rate of ice formation during the freezing process. Ice growing in super-cooled environments results in dendrites that resemble snowflakes in their morphology and which negatively affect foam morphology and structure. Moreover, freezing in non-controlled systems, where the NCC slurry is exposed to low temperatures from different directions, presents little control on the direction of ice crystals formation and as a result cross sections of randomly crystallized foams may display local orientation in planes that face different directions parallel to the direction of the formed ice crystals; consequently leading to inferior mechanical properties.
Production of composite foams made from NCC reinforced with bio-resins as core materials for sandwich composites applications was also described [11]. Such composites usually have higher densities and therefore may be of less applicability as composite materials of low weight and high-strength are better suitable and more desired.